This invention relates to a measuring apparatus, particularly a measuring apparatus equipped with a light source for illuminating a target portion.
2. Description of Related Art
There have been a variety of measuring apparatus equipped with a light source. For example, there has been a colorimeter which is constructed so that a target portion is illuminated with white light of a certain luminous intensity, and light reflected from the target portion is measured to obtain tristimulus values of the reflected light. Such a colorimeter is liable to give different measurement values depending on color temperatures of the light source. Accordingly, it is necessary to maintain the light source at a constant color temperature.
For example, color temperature of light which is emitted by a xenon flash lamp (hereinafter referred to as Xe tube) used as a light source of the colorimeter varies with the flash tube current which flows in the Xe tube when it flashes. The flash tube current is supplied from a charged main capacitor. The flash tube current is determined by the circuit impedance of a discharge loop, including the Xe tube, and the voltage applied across the Xe tube at the time of flashing. The impedance of the Xe tube is determined by its internal gas pressure and electrode spacing. Xe tubes have their respective specific impedance values, which vary with their temperatures. Therefore, the flash tube current varies with the lapse of time during a discharge process even when the lighting source voltage is kept constant. Furthermore, color temperature of the flash tube cannot be maintained at a constant level even when flashing is repeated at relatively short intervals. It is therefore difficult to maintain the light source at a constant color temperature when using a discharge lamp as the light source.
When using another type of light source, for example, an incandescent lamp, the color temperature rises until it reaches a specific level after the lamp is turned on. In this case, the rising speed and peak level of color temperature are subject to change with time even if the applied voltage is controlled to a fixed value. When the lamp is lit by a pulse voltage, color temperature also changes with variations in pulse duration.
Accordingly, in such measuring apparatus equipped with a light source for illuminating a target portion, it is very difficult to completely obtain a constant color temperature in each lighting process. Consequently, the measurement accuracy is lowered due to slight variations in color temperature.
It is an object of the present invention to provide a measuring apparatus which has overcome the above-mentioned drawbacks, and makes it possible to eliminate color temperature variations of the light source from influencing the measurement.